Regina L. Turetskaya

Redundancy in Tumor Necrosis Factor (TNF) and Lymphotoxin (LT) Signaling In Vivo: Mice with Inactivation of the Entire TNF/LT Locus versus Single-Knockout Mice

ABSTRACT Homologous genes and gene products often have redundant physiological functions. Members of the tumor necrosis factor (TNF) family of cytokines can signal activation, proliferation, differentiation, costimulation, inhibition, or cell death, depending on the type and status of the target cell. TNF, lymphotoxin α (LTα), and LTβ form a subfamily of a larger family of TNF-related ligands with their genes being linked within a compact 12-kb cluster inside the major histocompatibility complex locus. Singly TNF-, LTα-, and LTβ-deficient mice share several phenotypic features, suggesting that TNF/LT signaling pathways may regulate overlapping sets of target genes. In order to directly address the issue of redundancy of TNF/LT signaling, we used the Cre-loxP recombination system to create mice with a deletion of the entire TNF/LT locus. Mice with a triple LTβ/TNF/LTα deficiency essentially manifest a combination of LT and TNF single-knockout phenotypes, except for microarchitecture of the spleen, where the disorder of lymphoid cell positioning and functional T- and B-cell compartmentalization is severer than that found in TNF or LT single-knockout mice. Thus, our data support the notion that TNF and LT have largely nonredundant functions in vivo.

An autocrine growth factor constitutively produced by a human lymphoblastoid B-cell line is serologically related to lymphotoxin (TNF-β)

Conditioned medium of a human lymphoblastoid B-cell line RPMI-6410t contains a factor sufficient for maintainance and growth of these cells. At the same time RPMI-6410t cells secrete a soluble factor cytotoxic towards mouse L929 cells. Production of these activities by RPMI-6410t cell line and its subclones is significantly enhanced after activation with phorbol mirystate acetate (PMA). Both activities can be neutralized by antiserum raised against recombinant lymphotoxin (rTNF-beta) but not by antibodies against tumor necrosis factor (rTNF-alpha). Northern analysis showed the presence of lymphotoxin mRNA which is further induced after PMA treatment. These data suggest that both autocrine growth factor and cytotoxic activities correspond to the same molecule(s) probably identical to 25 kD lymphotoxin (TNF-beta).

Comparative Characterization of Lymphotoxin Transcripts from Human B- and T-Cell Lines, Peripheral Lymphocytes, and Normal Tissues

Membrane lymphotoxin (LT) is a heterotrimer LTα1β2 , and its production depends on two genes. Northern blotting was employed in studying their transcription in human B- and T-lymphoma cell lines and in peripheral blood lymphocytes before and after induction with phorbol myristate acetate (PMA). Transcription of either gene proved to be similarly regulated in several cell lines and in blood lymphocytes. Activation of the LTα gene was associated with induction of transcription factor NF-κB (p50/p65) upon cell treatment with PMA. On evidence of RT–PCR, two transcripts of the LTβ gene were present in equimolar amounts in all lymphoid cells. A product of alternative splicing contained an open reading frame coding for the cytoplasmic portion of LTβ.

Genes Encoding Tumor Necrosis Factors: Genome Organization, Polymorphism, and Expression

Tumor necrosis factor (TNF-α) and lymphotoxin (TNF-β) are two related cytokines sharing a broad spectrum of activities [1–4]. In particular, TNF-α is one of the principal mediators of inflammation [2]. TNFs are also involved in the control of hematopoiesis. TNF-α stimulates production of hematopoietic growth factors by fibroblasts, macrophages, and endothelial cells [5,6]. On the other hand, the same cytokine inhibits colony formation by hematopoietic progenitor cells in vitro [7,8]. In vivo, TNF-α stimulates formation of spleen colonies and exhibits an overall protective effect in sublethally irradiated mice [9, 10].